Fig. 81.
If we wish to denote the upper yellow line BD, [fig. 81], we can speak of it as the yellow γ line, meaning the yellow line which is separated from the primary yellow line by the red movement.
In a similar way each of the other squares has null points, red and yellow lines. Although the yellow square is all yellow, its line CD, for instance, can be referred to as its red line.
This nomenclature can be extended.
If the eight cubes drawn, in [fig. 82], are put close together, as on the right hand of the diagram, they form a cube, and in them, as thus arranged, a going up is represented by adding red to the zero, or null colour, a going away by adding yellow, a going to the right by adding white. White is used as a colour, as a pigment, which produces a colour change in the pigments with which it is mixed. From whatever cube of the lower set we start, a motion up brings us to a cube showing a change to red, thus light yellow becomes light yellow red, or light orange, which is called ochre. And going to the right from the null on the left we have a change involving the introduction of white, while the yellow change runs from front to back. There are three colour axes—the red, the white, the yellow—and these run in the position the cubes occupy in the drawing—up, to the right, away—but they could be turned about to occupy any positions in space.
Fig. 82.
Fig. 83.
We can conveniently represent a block of cubes by three sets of squares, representing each the base of a cube.